In the operation of a rotary offset press, an image is reproduced on a sheet of paper or some other print stock by a plate cylinder which carries the image, a blanket cylinder which has an ink transfer surface for receiving the inked image, and an impression cylinder which presses the paper against the blanket cylinder so that the inked image is transferred to the paper. In some applications, a protective and/or decorative coating is applied to the surface of the freshly printed sheets. The freshly printed sheets are then conveyed to a sheet delivery stacker in which the finally printed sheets are collected and stacked.
In many instances printing liquids applied at an intermediate printing station of a multi-stand printing press use water as a solvent, dilutent or vehicle. These may be applied as images, spot coatings or overall coatings prior to subsequent in-line lithographic printing. If too much water remains on the printed substrate, problems with delayed drying and image quality can occur because moisture inhibits drying of ink. The problems are exacerbated as press speed is increased.
The wet ink and coatings should be dried before the sheets are stacked or run back through the press for a second pass, to prevent smearing defects and to prevent offsetting of the ink on the unprinted side of the sheets as they are stacked. Spray powder has been applied between the freshly printed sheets which are to be stacked to improve sheet handling and to separate one delivered sheet from the next sheet to prevent offsetting while the ink and/or coating dries. One limitation of the use of spray powder is that fugitive particles of the spray powder disperse into the press room and collect on press equipment, causing electrical and mechanical breakdowns and imposing a potential health hazard for press room personnel.